1. Field of the Invention
This invention relates to a pipe joint, a method of making the pipe joint, and a method of constructing a branching connection. More specifically, this invention relates to a saddle type plastic pipe joint used for connecting a branching plastic pipe to a main plastic pipe, a method of making the saddle type pipe joint and a method of connecting a branching plastic pipe to a main plastic pipe by utilizing a shape-memorizing function of the pipe joint.
2. Description of the Related Art
In recent years, plastic pipes such as polyolefin pipes, for example, a polyethylene pipe are often used in substitution for metal pipes as water lines, hot water supply pipes and hot spring-conducting pipes. Polyolefin pipes have advantages, compared with metal pipes, such that they are light in weight and easy to handle and have good corrosion resistance and chemical resistance, a scale is not readily deposited thereon, and they are of a large length and thus the number of joints can be decreased.
In a pipe arrangement composed of plastic pipes, branching joints are indispensable for the construction of branching connections where pipes branch from midway locations of the length of a main pipe. As means for the construction of branching connections, a saddle type joint having embedded therein an electrically conductive wire as a heating element is used. The saddle type joint has a structure such that a circular opening is formed in the location of the main body where a branching pipe is connected to the main pipe, and the electrically conductive wire is wound in a spiral configuration around the opening. The saddle type joint has a saddle portion which covers approximately a half of the entire periphery of the main pipe (for example, see U.S. Pat. Nos. 4,455,482 and 4,894,521). The pipe joint of this type which has a spirally configured electrically conductive wire requires much labor to make the joint, and a special device or cool (for example, as illustrated in FIG. 12 of U.S. Pat. No. 4,452,482) is necessary for fitting the saddle-shaped part of the joint to the main pipe. If the saddle-shaped part is fitted to the main pipe by welding without use of the special device or tool, where the main pipe has a distortion even though only to a minor extent, gaps are left between the inner surface of the pipe joint and the outer surface of the main pipe with the result that a good fusion-bonding cannot be obtained.
A pipe joint having a heat-generating plastic element has also been proposed, which element has incorporated therein an electrically conductive filler instead of the electrically conductive wires. For example, in European Patent 157,640, a heat-recoverable pipe joint is disclosed which has a heat-recoverable element composed of a heat-generating polyethylene layer having incorporated therein a conductive filler and a crosslinked polyethylene layer formed on the heat-generating polyethylene layer. As an example of the application of the heat-recoverable pipe joint, a construction of a branching connection wherein a pipe branches from a main pipe (see FIGS. 5 and 6). This branching connection is achieved by a method wherein a hole is previously bored in the main pipe, and a pipe is inserted into the hole to construct a branched pipe. Therefore, the branching connection cannot be constructed while a liquid is allowed to flow through the installed main pipe. Further, although a special device or tool as described in U.S. Pat. No. 4,452,482 is not necessary in the heat-recoverable pipe joint, the joint is not firmly bound to the main pipe because the crosslinked polyethylene surface layer is relatively thin.
Recently, an electrofusion pipe joint has been proposed which comprises a base body composed of a thermosetting polynorbornene resin and molded by a reaction molding, and a heat-generating member composed of a plastic having incorporated therein an electrically conductive filler (see Japanese Unexamined Patent Publication No. H4-294115). In this publication, pipe joints of a complicated or special shape such as elbow-type and cheese-type joints, as well as a simple pipe-type joint, are described. These joints of a complicated or special shape are made by a process wherein a hollow plastic article having not incorporated therein an electrically conductive filler is blow-molded, a heat-generating element is fitted to a desired site of the hollow article, and then, a norbornene monomer is cast and polymerized in a mold where the heat-generating element-fitted hollow article has been placed as a core. However, this process cannot be employed for making non-hollow pipe joints such as a saddle-type pipe joint. Further, the heat-generating element having a hole in the connection site as described in the abovementioned U.S. Pat No. 4,455,482 is difficult to make by injection-molding a plastic material having incorporated therein an electrically conductive filler because the filler-incorporated plastic material has a poor melt-flowability and further a complicated mold having fitted thereto a combination of a plurality of cores is necessary.
If the saddle-shaped part of a saddle type pipe joint which is fusion-bonded to a main pipe is made from a plastic material having not incorporated therein an electrically conductive filler, and a heat-generating element having an electrically conductive filler is fitted onto the saddle-shaped part, then a heat-generating element of a special shape which has a hole at the site of a pipe-branching connection must be used, and further, the fusion-bonding of the saddle-shaped part to the main pipe is not satisfactory in the peripheral area of the branching pipe-connected part because the conductive filler is not present at the site of branching connection.